The Optimum Piston Position for Wide-Band Coaxial-to-Waveguide Transducers

A coaxial line can be matched to a waveguide by means of a probe antenna located ahead of a short-circuiting plunger. An impedance match can usually be achieved by varying any two of the following three dimensions: (a) the off-center position of the probe, (b) the probe length, (c) the piston position. This paper points out that there is, theoretically, an optimum piston position for greatest bandwidth, and presents some evidence corroborating this theory. Bandwidths greater than ±10 per cent to the 1 db swr points have been realized by fixing the piston at its optimum position and varying (a) and (b) above to obtain a match.